The invention generally relates to inside-out nuclear magnetic resonance (NMR) measurements, and more particularly, the invention relates to detecting tool motion effects on NMR measurements of formation properties surrounding a borehole, such as measurements of the hydrogen content of the formation, for example.
Referring to FIG. 1, as an example, nuclear magnetic resonance (NMR) measurements may be obtained in a logging while drilling (LWD) operation to map the properties of a subterranean formation 10. In this manner, an axisymmetric NMR tool 6 may be part of a drill string 5 that is used to drill a borehole 3 in the formation 10. The tool 6 may be, as examples, one of the tools described in Sezginer et. al., U.S. Pat. No. 5,705,927, entitled, "Pulsed Nuclear Magnetism Tool For Formation Evaluation While Drilling Including a Shortened or Truncated CPMG Sequence," granted Jan. 6, 1998; Miller, U.S. Pat. No. 5,280,243, entitled, "System For Logging a Well During the Drilling Thereof," granted Jan. 18, 1994; Taicher et. al., U.S. Pat. No. 5,757,186, entitled, "Nuclear Magnetic Resonance Well Logging Apparatus and Method Adapted for Measurement-While-Drilling," granted May 26, 1998; Jackson et. al., U.S. Pat. No. 4,350,955, entitled, "Magnetic Resonance Apparatus," granted Sept. 21, 1982; U.S. patent application Ser. No. 09,186,950, entitled, "Apparatus and Method for Obtaining a Nuclear Magnetic Resonance Measurement While Drilling," filed on Nov. 5, 1998; or Prammer et. al., WO99/36801 entitled "Method and Apparatus for Nuclear Magnetic Resonance Measuring While Drilling," published on Jul. 22, 1999.
The NMR measuring process is separated by two distinct features from most other downhole formation measurements. First, the NMR signal from the formation comes from a small resonance volume, such as a generally thin resonance shell 20a(see FIG. 2), and the resonance volume 20a has a radial thickness that is proportional to the magnitude of an oscillating magnetic field and inversely proportional to the gradient of a static magnetic field. Depending on the shape of the resonance zones, the volume extends, as an example, from as little as 1 millimeter (mm.) in one direction and as long as several inches in another. Secondly, the NMR measurement may not be instantaneous. Both of these facts combined make the NMR measurements prone to tool motions, such as the motion that is attributable to the movement of the NMR tool 6 around the periphery of the borehole 3, as further described below.
The NMR tool 6 measures T2 spin-spin relaxation times of hydrogen nuclei of the formation 10 by radiating NMR detection sequences to cause the nuclei to produce spin echoes. The spin echoes, in turn, may be analyzed to produce a distribution of T2 times, and the properties of the formation may be obtained from this distribution. For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence 15 that is depicted in FIG. 3. By applying the sequence 15, a distribution of T2 times may be obtained, and this distribution may be used to determine and map the properties of the formation 10.
A technique that uses CPMG sequences 15 to measure the T2 times may include the following steps. The NMR tool 6 pulses the B.sub.1 field for an appropriate time interval to apply a 90.degree. excitation pulse 14a to rotate the spins of hydrogen nuclei that are initially aligned along the direction of the B.sub.0 field. Although not shown in detail, each pulse is effectively an envelope, or burst, of a radio frequency RF carrier signal. When the spins are rotated around B.sub.1 away from the direction of the B.sub.0 field, the spins immediately begin to precess around B.sub.0. The pulse is stopped when the spins are rotated by 90.degree. into the plane perpendicular to the B.sub.0 field. They continue to precess in this plane first in unison, then gradually losing synchronization. At a fixed time T.sub.CP following the excitation pulse 14a, the NMR tool 6 pulses the B.sub.1 field for a longer period of time (than the excitation pulse 14a) to apply an NMR refocusing pulse 14b to rotate the precessing spins through an angle of 180.degree. with the carrier phase shifted by .+-.90.degree.. This step may be repeated "k" times (where "k" is called the number of echoes and may assume a value anywhere from several to as many as several thousand, as an example) at the interval of 2.multidot.T.sub.CP. The NMR pulse 14b causes the spins to resynchronize and radiate an associated spin echo signal 16 (see FIG. 4) that peaks at a time called T.sub.CP after the 180.degree. refocusing NMR pulse 14b. After completing the spin-echo sequence, a waiting period (usually called a wait time) is required to allow the spins to return to equilibrium along the B.sub.0 field before starting the next CPMG sequence 15 to collect another set of spin echo signals. The decay of each set of spin echoes is observed and used to derive the T2 distribution.
One way to identify potential problems caused by motion effects requires the use of a motion detection device, such as a strain gauge, an ultrasonic range finder, an accelerometer or a magnetometer. In this manner, the motion detection device is used to establish a threshold for evaluating the quality of the NMR measurement. Such an arrangement is described in PCT Application Number PCT/US97/23975, entitled, "Method for Formation Evaluation While Drilling," that was filed on Dec. 29, 1997. However, conventional motion detection devices may not specifically indicate desired corrections to the measurement data to compensate for tool motion.
Thus, there is a continuing need for a method to more precisely detect tool motion effects on NMR measurements. There is also a continuing need for a method to adapt NMR measurement analysis in response to the detected tool motion effects.